Process for the manufacture of nanocomposites

ABSTRACT

Process for the manufacture of nanocomposites comprising at least one lamellar clay and at least one polymer, according to which the clay, the polymer and a bifunctional solvent simultaneously comprising an organophilic group capable of dissolving the polymer and a hydrophilic group capable of reacting with the clay are brought into contact.

The present invention relates to a process for the manufacture ofcompositions comprising a lamellar clay and a polymer.

In recent years, much research effort has been invested in thedevelopment of nanostructured materials and in particular ofnanocomposites, that is to say of polymers reinforced with a filler ofnanoscopic size (that is to say, at least one of the characteristicdimensions of which is of the order of a nanometre, indeed even of tensof nanometres). Such composites find their application in variousfields, such as the motor vehicle industry, packaging, flame retardancy,and the like. They exhibit noteworthy properties due to the size and tothe high anisotropy of the nanoscopic fillers.

Such fillers are potentially available (that is to say, available withappropriate treatment) in nature in the form of lamellar clays. However,it is advisable to be able to insert polymer chains between the clayplatelets and even, preferably, to exfoliate the clay (that is to say,to separate its platelets) in order for it to be able to actually createan ordered structure on the nanoscopic scale. This is because thecharacteristic dimension on the nanometric scale is, in the case oflamellar fillers, their thickness, provided, of course, that they can beisolated from the aggregates which they naturally form and which arelarger in size. In point of fact, clays are hydrophilic substanceswhereas the majority of polymers are hydrophobic substances. Thus, toensure that polymer molecules penetrate into the lamellar structure ofthe clay and break it up generally requires a modification of the clayin order to render it more organophilic.

Thus, in the literature, techniques have been provided which makepossible either the modification of the clay with appropriate compounds(typically, organic surfactants of ammonium or phosphonium type,silanes, and the like) prior to its incorporation into a polymer matrix(in particular during melt processing) or the in situ modification ofthe clay (during its melt processing) by at least one appropriatesubstance (swelling agent). Other techniques, such as the polymerizationof certain monomers in situ in the modified clay or the directincorporation of polymers in solution between the clay plateletssubsequent to ion exchange phenomena, have also been provided. Thesetechniques generally exhibit the disadvantage of involving a stage ofpretreatment of the clay with agents which are generally expensiveand/or specific to a given polymer (generally compatible with aqueousmedia).

Thus, Patent Application WO 00/78540 discloses the use of clays modifiedwith a surfactant (salt of “onium” type) of given formula for themanufacture of nanocomposites using processes in an extruder (melt) orin solution (in conventional solvents). However, this technique islimited to polymers which are readily inserted into the lamellarstructure of the clay. In addition, it requires recourse to a separatestage of treatment with specific surfactants.

With regard to U.S. Pat. No. 6,057,396 and U.S. Pat. No. 6,271,297, theydisclose a technique which consists in swelling the clay using aswelling agent which preferably comprises a monomer capable ofpolymerizing in situ between the platelets of the clay during a“reactive” extrusion. This technique is therefore not applicable to themanufacture of any type of nanocomposite since the polymer which haspolymerized in situ must be identical to or compatible with that of thematrix of the nanocomposite.

It is an object of the present invention to provide a process whichmakes it possible to easily obtain nanocomposites comprising a polymerand a lamellar clay impregnated with and/or exfoliated by the polymerwithout having to modify the clay beforehand, in particular withexpensive surfactants, and/or to be limited in the choice of the polymermatrix (as in the case of the reactive extrusion processes).

The present invention consequently relates to a process for themanufacture of nanocomposites comprising at least one lamellar clay andat least one polymer, according to which the clay, the polymer and abifunctional solvent simultaneously comprising an organophilic groupcapable of dissolving the polymer and a hydrophilic group capable ofreacting with the clay are brought into contact. The term “hydrophilicgroup capable of reacting with the clay” is intended to denote thefunctional groups which can interact with the clay and can render itorganophilic.

The clay (or inorganic compound based on silica, on alumina and onwater) used in the process according to the present invention is alamellar clay (phyllitic clay). The majority of clays are lamellar andthey are generally classified into 3 families: that of kaolin, of micaand of montmorillonite. The clays of the final fly, such asmontmorillonite proper, beidelite, nontronite, hectorite, saponite andsauconite, also known as smectites, are particularly well suited to theprocess according to the present invention, montmorillonite itself beingpreferred.

The polymer used in the process according to the present invention canbe of any type. It can be a thermoplastic or thermosetting polymer, ahomopolymer or a random or block copolymer, and the like. The processaccording to the present invention is particularly well suited tohalogenated polymers, for which the conventional methods of clayimpregnation/exfoliation by means of a solvent are not suitable (asineffective and/or excessively lengthy). In particular, PVC (poly(vinylchloride)) and PVDF (poly(vinylidene fluoride)) resins give goodresults, the latter being particularly preferred. Both homopolymers andcopolymers of vinyl chloride, mainly with methyl metacrylate, give goodresults in the case of PVC. With regard to PVDF, which is particularlypreferred, both homopolymers (of vinylidene fluoride) and copolymerswith vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene,tetrafluoroethylene, hexafluoropropylene or ethylene give good results.It should also be noted that the process according to the presentinvention also gives good results with biodegradable polymers (such aspolycaprolactone) and aromatic polyamides (polyarylamides). Finally,several compatible polymers (that is to say, miscible within a widerange of proportions) can be used in the process according to thepresent invention.

The term “bifunctional solvent” is intended in fact to denote a solventhaving at least two functional groups, one of which makes it possible todissolve the polymer and the other of which makes it possible tointeract with the clay and to render it organophilic. The bifunctionalsolvent used in the process according to the present invention isgenerally an organic solvent obtained by substituting a solvent for thechosen polymer with a hydrophilic moiety compatible with the clay, suchas an alcohol and/or an amine, for example. In the case of halogenatedpolymers, which exhibit a high affinity for polar organic solvents, sucha solvent will advantageously be substituted, preferably, with at leastone alcohol. Bifunctional solvents which have given good results, inparticular with halogenated polymers, are the organic solvents includinga polar heterocycle substituted by one or more groups crying one or morealcohol functional groups. These substituents can be short-chain (C₁ toC₄) monoalcohols and/or can be C₁-C₈ polyols and/or polymeric polyols(generally with a maximum of 50 monomer units). They are preferably(derivatives of) methanol or ethanol, i.e. hydroxymethyl or hydroxyethylgroup. Molecules comprising furan, tetrahydrofuran (H) or pyrrolidone aspolar heterocycle are highly suitable. Furfuryl alcohol andtetrahydrofurfuryl alcohol (furan or THF substituted by hydroxymethylgroup) give excellent results, in particular in the case of PVC and ofPVDF. In addition, they are common and inexpensive solvents.

In some cases, the solubility of the polymer in the contact medium canbe further improved by incorporating therein a monofunctional(unsubstituted) solvent fox the polymer. Thus, advantageously, thebifunctional solvent will be used as a mixture with a monofunctionalsolvent for the polymer. In the case of halogenated polymers, theaddition of cyclohexanone to the abovementioned solvents gives goodresults. It is also possible to seek to further increase the solventeffect by any means generally known for this purpose, for example withvigorous stirring, with ultrasonic radiation, and the like.

The clay, the polymer and the bifunctional solvent can be brought intocontact by introducing all the ingredients at the same time into thecontact medium, either separately, in any order, or alternatively invarious combinations. Thus, first the clay can be treated with thebifunctional solvent (in order to cause it to swell) and then thepolymer (optionally in solution also in a solvent, which may or may notbe bifunctional) can be added to the mixture. Alternatively, first thepolymer can be dissolved in the bifunctional solvent and subsequentlythe clay can be incorporated in the solution obtained. The latter methodgives good results.

The temperature and the duration of the operation in which the clay, thepolymer and the bifunctional solvent are brought into contact are chosenaccording to the nature of the polymer and the parameter which it isdesired to optimize. If it is desired to rapidly obtain (for exampleafter a few hours or a few days) the penetration of the polymer into theclay and/or its exfoliation, it is possible, for example, to heat Thisis because the fact of operating at a higher temperature (for example at80° C. or more instead of at ambient temperature) makes it possible toaccelerate the diffusion phenomena. However, it should be pointed outthat in some cases (according to the nature of the polymer and of thebifunctional solvent which are chosen) a high temperature can promotedecomposition of the polymer. For the purpose of limiting thisdecomposition, the heating will preferably be limited. Thus, it will bepreferable to operate at a lower temperature, for example at ambienttemperature, which will generally involve, however, an increase in theduration of contact (which may then reach several weeks, indeed evenseveral months).

Finally, for the purpose in particular of limiting the decomposition ofthe polymer, it may prove to be advantageous to carry out the operationin which the clay, the polymer and the bifunctional solvent are broughtinto contact under a controlled atmosphere (inert gas) and/or in thepresence of stabilizers for the polymer and/or in the presence of acosolvent which is inert with respect to the polymer, such assupercritical CO₂.

The amount of solvent(s) used is preferably suited to the amount ofpolymer to be dissolved and to the amount of clay to be exfoliated (orimpregnated).

Generally, use is made of an amount of solvent(s) at least equal to 5times that of the polymer by weight and preferably at least equal to 10times that of the polymer by weight. However, care will advantageouslybe taken not to exceed the amount of solvent(s) strictly necessary forthe satisfactory progression of the process, given in particular thatthe solvent is often removed before the final use of the nanocomposite.

With regard to the polymer/clay proportion in the nanocomposite, thisproportion depends on the use targeted. In the majority of the finalapplications, a concentration of clay of the order of a few % by weightis sufficient. It is in fact rare for the concentration of clay toexceed 10% by weight of the nanocomposite. Consequently, when the finalobject targeted can be obtained directly from theclay/polymer/bifunctional solvent mixture (for example by simpleevaporation of the latter at high temperature, which also has the effectof melting the polymer), the polymer/clay proportion will advantageouslybe greater than or equal to 10:1, indeed even greater than or equal to20:1, by weight. In contrast, in the case where the compositionaccording to the present invention acts as concentrate to be dilutedduring the melt processing of virgin polymer or during a polymerization,this proportion can become less than or equal to 3:2, indeed even lessthan or equal to 1:1, by weight.

As mentioned above, in the process according to the present invention,the operation in which the clay, the polymer and the bifunctionalsolvent are brought into contact can be followed:

-   -   by a stage of evaporation of the bifunctional solvent;    -   by a stage of separation of the clay and of the polymer, on the        one hand, and of the bifunctional solvent, on the other hand, by        any appropriate means (filtration, centrifugation, and the        like);    -   by direct use of the mixture comprising the polymer, the clay        and the bifunctional solvent in a process for the polymerization        of a polymer identical to or compatible with that which has        impregnated and/or exfoliated the clay.

The nanocomposite obtained by the process according to the presentinvention can be subjected to a melt processing, optionally inconjunction with a polymer identical to or compatible with that whichhas impregnated and/or exfoliated the clay, this being done in order tomanufacture components (finished objects) of given form.

During this processing, care will preferably be taken to adapt the shearconditions so as to obtain a homogeneous distribution of the clayplatelets in the polymer matrix.

The present invention is illustrated without implied limitation by theexamples and counterexamples described in the following paragraphs. Inthese, use was made:

-   -   of Bavarian montmorillonite available from IKO        (Industriemineralien und Kohlenstoffe GmbH & Co), based on        Na/SiMgAl/Fe (4.5%), which can comprise, by way of impurities,        traces of silicas and of micas,    -   as PVC, of the Solvic® S266RC grade available from Solvay,    -   as PVDF, of the Solef® 1010 grade, also available from Solvay.

COMPARATIVE EXAMPLE 1 (NOT IN ACCORDANCE WITH THE INVENTION)

1 g of PVC was dissolved in 15 ml of cyclohexanone under hot conditions(150° C.). 0.25 g of PEO (poly(ethylene oxide)) and 0.5 g ofmontmorillonite were subsequently added to the solution and the combinedmixture was placed under magnetic stirring. After 18 h, the suspensionwas filtered and the solid residue was examined by electron microscopy.

Impregnation of the lamellar structure of the montmorillonite by thepolymer was observed but no exfoliation (breakup) of this structure wasobserved.

EXAMPLE 2 (IN ACCORDANCE WITH THE INVENTION)

1 g of PVC was dissolved in a mixture of 5 ml of cyclohexanone and of 5ml of furfuryl alcohol under hot conditions (80° C.). 0.5 g ofmontmorillonite was subsequently added to the solution and the combinedmixture was placed under magnetic sting. After 18 h, the suspension wasfiltered and the solid residue was examined by electron microscopy.

Impregnation of the lamellar structure of the montmorillonite by thepolymer was observed and initiation of exfoliation (breakup) of thisstructure was also observed.

EXAMPLE 3 (IN ACCORDANCE WITH THE INVENTION)

Example 2 was repeated but multiplying the amounts of reactants by 30and extending the test for a total duration of 6 days.

Not only impregnation but also complete exfoliation of the lamellarstructure of the montmorillonite are then observed.

EXAMPLE 4 (IN ACCORDANCE WITH THE INVENTION)

1 g of PVDF was dissolved in a mixture of 5 ml of cyclohexanone and of 5ml of furfuryl alcohol under hot conditions (150° C.). 0.5 g ofmontmorillonite was subsequently added to the solution and the combinedmixture was placed under magnetic stirring. After 18 h, the suspensionwas filtered and the solid residue was examined by electron microscopy.

Impregnation of the lamellar structure of the montmorillonite by thepolymer was observed and initiation of exfoliation (breakup) of thisstructure was also observed.

EXAMPLE 5 (IN ACCORDANCE WITH THE INVENTION)

Example 4 was repeated but multiplying the amounts of reactants by 30and extending the test for a total duration of 6 days.

Not only impregnation but also complete exfoliation of the lamellarstructure of the montmorillonite are then observed.

1-10. (canceled)
 11. A process for the manufacture of nanocompositescomprising at least one lamellar clay and at least one polymer, whereinthe clay, the polymer, and a bifunctional solvent simultaneouslycomprising an organophilic group capable of dissolving the polymer and ahydrophilic group capable of reacting with the clay are brought intocontact.
 12. The process of claim 11, wherein the lamellar clay is amontmorillonite.
 13. The process of claim 11, wherein the polymer is ahalogenated polymer.
 14. The process of claim 13, wherein the polymer isa PVDF resin.
 15. The process of claim 11, wherein the bifunctionalsolvent is an organic solvent including a polar heterocycle substitutedby one or more groups comprising one or more alcohol functional groups.16. The process of claim 15, wherein the bifunctional solvent isfurfuryl alcohol or tetrahydrofurfuryl alcohol.
 17. The process of claim11, wherein the bifunctional solvent is used as a mixture with amonofunctional solvent for the polymer.
 18. The process of claim 1 1,wherein the operation in which the clay, the polymer and thebifunctional solvent are brought into contact takes place under acontrolled atmosphere and/or in the presence of stabilizers for thepolymer and/or in the presence of supercritical CO₂.
 19. The process ofclaim 11, wherein the operation in which the clay, the polymer and thebifunctional solvent are brought into contact is followed by a stage ofevaporation of the bifunctional solvent; by a stage of separation of theclay and of the polymer, on the one hand, and of the bifunctionalsolvent, on the other hand; or by direct use of the mixture comprisingthe polymer, the clay and the bifunctional solvent in a process for thepolymerization of a polymer identical to or compatible with that whichhas impregnated and/or exfoliated the clay.
 20. The process of claim 19,wherein the nanocomposite is subjected to melt processing, optionally inconjunction with a polymer identical to or compatible with that whichhas impregnated and/or exfoliated the clay.